This invention relates to a Doppler target tracking system of the type in which fixed ground based transmitters transmit continuous wave signals to be reflected from a moving target and in which a moving platform, such as an aircraft carries a receiver to receive the reflected Doppler shifted signals and more particularly to such a system in which ground clutter is suppressed or canceled.
In Doppler target tracking systems, the received signals include signals reflected from the moving target as well as signals reflected from ground based structures. The reflection from the ground based structures is called "ground clutter", which hereinafter is referred to simply as "clutter". The present invention is directed to a continuous wave Doppler system, in which the clutter components in the Doppler signal received by the moving platform are characterized so that they can be distinguished from target signals. In Doppler systems employing an aircraft or other moving platform to receive the reflected signals, the clutter signals are Doppler shifted by the motion of the aircraft which receives the reflected signals. The clutter signals are much in excess of signals reflected from moving targets which are desired to be tracked and the Doppler shift occurs in the same region of Doppler space that the Doppler shift from the targets appears. Accordingly, to enable efficient detection and tracking of a target, the clutter signals need to be effectively distinguished from target signals.
Prior to the present invention, the problem of clutter in Doppler radar systems have been recognized and systems have been developed to eliminate the ground clutter from Doppler radar signals. These systems are for the most part designed for use with pulse type radar and for the most part are not designed for use in continuous wave type radar systems. Some systems of the prior art are intended to be applicable to all kinds of Doppler radar systems, including continuous wave systems, but these systems of the prior art would not be effective in a system in which the reflected signals are received by a moving platform such as an aircraft.